Shape memory materials (particularly those of nitinol and various nitinol like alloys) have physical properties that may be varied dependent upon the temperature of the immediate environment in which the memory device exists. For nitinol alloys two forms are described: Martensite, which is the low temperature form generally may be thought of as a malleable metal; and Austenite, a high temperature form which exhibits properties of shape memory and super-elasticity. The transition between two forms of nitinol is characterized by a hysteresis curve which is defined by complete transition to martensite form (Mf) at the lower end of the temperature range and complete transition to austenite (Af) at the higher end of temperature range. The upper and lower limits of and breadth of this range depend upon alloy composition of the shape memory material and upon work performed on the device in the manufacturing process.
Alloys are available with Mf temperatures above 0 degrees centigrade and Af temperatures below 37 degrees centigrade; devices with these characteristics may be positioned at temperatures at or below Mf and deployed by heating to temperature ranges equal to or greater than Af. There is a need to be able to reliably deploy a device composed of shape memory material with a hysteresis curve within this defined range into the human or mammalian body.
Current methods of stent placement invariably involve introduction of a pre-formed stent across an obstructing lesion with limitation in the size and geometry of the stent mandated by its final form as the devices must be placed with a size nearly as great as their final deployed size. Current stents may be expanded to a limited degree with balloon instrumentation however expansion is limited to less than a fifty percent increase over the placement dimension, furthermore the capacity for balloon expansion comes at the cost of having the stent proper composed of a relatively malleable material which may be subject to compression and re-stenosis with expansion of the obstructing lesion over time.
Multiple current systems exist and are in development for stenting in vascular and coronary artery disease states. Typical applications include use of uniform cross-section tubular stents for stenotic peripheral vascular lesions and most commonly for coronary artery lesions. Instrumentation techniques for these lesions typically involve introduction of a balloon across an obstructing lesion, inflation of the balloon affecting dilatation of the obstruction, followed by placement and release of a mechanically trapped stent across the lesion. While these techniques and devices have afforded good clinical results, there remain limitations in anatomic locations where stents may be placed and the precision with which they may be positioned.
Relative size of pre-deployment and post deployment shapes likewise becomes a limiting factor in use of currently available designs and placement techniques. There is a need for stent type devices that can be placed and precisely positioned across access lumens that are markedly smaller than the relative size of the deployed device or through convoluted routes of access that render placement of current devices technically difficult.
Billiary system disease often presents with symptoms of stenosis and or obstruction. Historically a number of techniques have been developed to affect decompression or drainage of an obstructed billiary tract; these techniques have varied from open surgical techniques to percutaneous drainage utilizing radiographic localization to a number of endoscopic techniques utilizing balloon dilatation and or plastic and metal stents.
Limitations of current techniques are largely due to re-obstruction secondary to overgrowth of tumor, accumulation of “bio-film”, or mechanical failure of the device maintaining patency for billiary drainage. Furthermore, current systems are either permanent in placement or replaced only with extreme difficulty.
Current stent designs accommodate differing sizing needs through a system of individual stents each of differing lengths. The operator must accurately choose the appropriate length of stent based upon information that cannot be directly measured; of necessity sizing information is gathered indirectly with fluoroscopic imaging or direct visualization through an endoscope with both methods subject to inaccuracies. Further, this system requires that an endoscopy suite must stock a large inventory of stents to meet patient requirements. No current billiary stenting systems are available having variable length capabilities.